Method and apparatus for adding reducing agent to secondary overfire air stream

ABSTRACT

A combustion boiler for burning fuel and producing heat to generate steam. A method of minimizing discharging of nitrogen oxides from a combustion boiler comprising the steps of spraying a reducing agent into an overfire air stream and supplying the overfire air stream to the combustion boiler. Vaporizing the sprayed reducing agent at least within about 0.1 seconds of the sprayed reducing agent entering the combustion boiler and reacting the vaporized reducing agent with the nitrogen oxides within the combustion chamber to reduce the nitrogen oxides and minimize discharge of nitrogen oxides from the combustion boiler. The reducing agent is nearly substantially instantaneous evaporated/gasified by the high energy of the reducing agent injection system

This application claims the benefit of provisional application No.60/501,452 filed Sep. 9, 2003.

FIELD OF THE INVENTION

The present invention relates to a combustion boiler having a reducingagent added to an overfire air stream such that the reducing agent isnearly substantially instantaneous evaporated/gasified by the highenergy of the reducing agent injection system. The present inventionalso relates to a combustion boiler having at least one initial overfireair duct partitioned into an initial primary overfire air stream and aninitial secondary overfire air stream, with a reducing agent being onlyadded to the initial secondary overfire air stream but not to theinitial primary overfire air stream.

BACKGROUND OF THE INVENTION

It is known in the prior art to add a reducing agent, such as ammoniafor example, to the combustion byproducts within a combustion boilerprior to the combustion byproducts exiting from the combustion boiler inorder to reduce the amount of nitrogen oxides remaining in the exhauststream as the exhaust stream leaves an exit section of the combustionboiler. The reducing agent is generally dispersed in the upper region ofthe combustion boiler and allowed to react with the combustionbyproducts prior to the combustion byproducts exiting via the exitsection. One method of applying a reducing agent to the combustionbyproducts of a combustion boiler is disclosed in U.S. Pat. No.4,902,488 while an alternative method is disclosed in U.S. Pat. No.6,280,695.

As used in the specification and the appending claims, the terms“nitrogen oxides” and “NO_(x)” are used interchangeably to refer to thenitric oxide (NO) and the nitrogen dioxide (NO₂) chemical species. Otheroxides of nitrogen, such as N₂O, N₂O₃, N₂O₄ and N₂O₅, are known butthese species are not emitted in significant quantities from stationarycombustion sources (except for possible N₂O). Thus, while the term“nitrogen oxides” can be used more generally to encompass all binary N—Ocompounds, it is used herein to refer in particular to the NO and NO₂(e.g., NO_(x) species).

While it is known to apply a reducing agent to the combustion byproductsprior to the combustion byproducts leaving the exit section of thecombustion boiler, the prior art methods heretofore have not achieved amaximum reduction in the amount of nitrogen oxides contained within theexhaust stream while also minimizing usage of the reducing agent.

Moreover, while it is generally known, according to U.S. Pat. No.6,280,695, to inject a reducing agent through overfire air ports into acombustion boiler, this known technique creates reducing agent dropletsor particles of a sufficient large size and sufficiently shields thereducing agent droplets in the overfire air stream to delay evaporationand/or gasification of the same so that the lifetime of the reducingagent droplets or particles is greater than the overfire air mixing timewith the combustion flue gases. While this technique may reduce somewhatthe discharge of nitrogen oxides, a still further reduction in theamount of nitrogen oxides contained within the exhaust stream isrequired while also minimizing usage of the reducing agent.

SUMMARY OF THE INVENTION

Wherefore, it is an object of the present invention to overcome theabove noted drawbacks associated with the prior art systems.

Another object of the present invention is to provide a combustionboiler and associated method for reducing the amount of nitrogen oxidescontained in the exhaust stream of the combustion boiler as that exhauststream leaves the exit section of the combustion boiler.

A further object of the present invention is to evaporate, gasify and/orvaporize the reducing agent, within an air overfire stream, to ensurethat the gasified or vaporized reducing agent intimately, rapidly andquickly reacts with the combustion byproducts, prior to the combustionbyproducts exiting the combustion boiler, to maximize the amount ofreduction of the combustion byproducts occurring within the combustionboiler and minimize the amount of nitrogen oxides, and other harmfulbyproducts contained within the exhaust stream, exiting from thecombustion boiler.

Yet another object of the present invention is 1) to provide an excessquantity of oxygen, via initial primary and secondary overfire airstreams, to the combustion area to insure that virtually all of the fuelsupply to the combustion boiler is totally and completely consumedwhile, at the same time, 2) to provide an ample supply of the reducingagent, via the initial secondary stream air stream, to maximizereduction of the nitrogen oxides contained in the exhaust immediatelyprior to the exhaust stream exiting from the combustion boiler via theexit section.

A still further object of the present invention is to divide the initialoverfire air stream into two separate streams, with the initial primaryoverfire air stream being exclusively overfire air, i.e., no reducingagent is added or mixed therewith, and the initial primary overfire airstream is designed to surround, encase and/or envelope the fuel andcombustion components supplied to the combustion boiler to ensurecomplete combustion thereof. The initial primary overfire air streamsupplies additional oxygen to the fuel combustion components, of thecombustion boiler, to facilitate substantially complete combustion andconsumption of substantially all of the fuel combustion components priorto the combustion byproducts exiting from the combustion boiler. Theinitial secondary overfire air stream contains a vaporized, evaporated,and/or gaseous reducing agent, along with additional oxygen, whichsurrounds, encases and/or envelopes both the initial primary overfireair stream and the fuel components so as to maximize reduction of thenitrogen oxides exhausted from the combustion boiler.

Yet another object of the present invention is to supply the reducingagent, in liquid form, to a heated initial secondary overfire air streamso that the energy and/or heat from the heated initial secondaryoverfire air stream substantially instantaneously vaporizes, evaporates,and/or gasifies the liquid reducing agent to enhance the reduction ofthe nitrogen oxides contained within the combustion boiler.

A still further object of the present invention is to supply thereducing agent, in liquid form, to an overfire air stream andsufficiently intermix the liquid reducing agent with the overfire airstream so that the liquid reducing agent is sheared and further brokendown into smaller particles which are rapidly heated and/or absorbenergy from the overfire air stream so as to be substantiallyinstantaneously vaporized or gasified either immediately upon, orshortly after, entering into the combustion boiler, i.e., the liquidreducing agent is substantially completely evaporated or gasified within0.01 seconds after entering into the combustion boiler, so that thevaporized or gasified reducing agent is immediately available to reactwith any nitrogen oxide(s) contained within the combustion boiler.

Another object is to prevent the liquid reducing agent from collectingor combining with one another to form larger droplets which will notsubstantially instantaneously vaporize or gasified upon entering intothe combustion boiler, i.e., within 0.01 seconds after entering into thecombustion boiler, and be immediately available for reaction with anynitrogen oxide(s) contained within the combustion boiler.

The present invention also relates to a method of minimizing dischargeof nitrogen oxides from a combustion boiler, the method comprising thesteps of: spraying a reducing agent into an overfire air stream;supplying the overfire air stream with the sprayed reducing agent to thecombustion boiler; vaporizing the sprayed reducing agent at least withinabout 0.1 seconds of the sprayed reducing agent entering the combustionboiler; and reacting the vaporized reducing agent with the nitrogenoxides within the combustion chamber to reduce the nitrogen oxides andminimize discharge of nitrogen oxides from the combustion boiler.

The present invention also relates to a combustion boiler for combustingfuel and generating heat, the combustion boiler comprising: a housingdefining an internal combustion chamber therein; at least one fuelsupply duct connected to the combustion boiler for supplying fuel to thecombustion chamber; and at least one initial overfire air duct forsupplying an initial overfire air stream to the combustion chamber tofacilitate complete combustion of the fuel supplied to the combustionboiler; wherein the at least one initial overfire air duct is dividedinto an initial primary overfire air stream and an initial secondaryinitial overfire air stream, and a reducing agent is added only to theinitial secondary initial overfire air stream, but not the initialprimary initial overfire air stream, prior to the secondary initialoverfire air stream discharging into the combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIG. 1 is a diagrammatic view of an improved combustion boiler accordingto the present invention;

FIG. 2 is a diagrammatic cross-sectional view of the combustion boiler,taken along section line 2-2 of FIG. 1, showing a plurality of initialoverfire air ducts;

FIG. 3 is a diagrammatic cross-sectional view of the combustion boiler,taken along section line 3-3 of FIG. 1, showing a plurality of primarycombustion chamber overfire air ducts;

FIG. 4 is a diagrammatic cross-sectional view of the combustion boiler,taken along section line 4-4 of FIG. 1, showing a plurality of secondarycombustion chamber overfire air ducts;

FIG. 5 is a diagrammatic cross-sectional view on one of the initialoverfire air ducts according to the present invention; and

FIG. 6 is a diagrammatic cross sectional view of an improved combustionboiler, according to the present invention, having a high energyinjector port.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the Figures, a brief description concerning a combustionboiler will first be provided and this will be followed by a detaileddescription of the present invention. As can be seen in those Figures,the combustion boiler is generally designated with reference numeralnumber 2. The combustion boiler 2 includes a base wall 4 and a sidewall,e.g., generally four planar sidewalls, as well as a top wall 8. The basewall 4, the four sidewalls 6 and the top wall 8 define an enclosedhousing 10 of the combustion boiler 2. An inwardly tapering indentation14 is formed in the rear sidewall 6 of the enclosed housing 10 and thisinwardly tapering indentation 14 forms a constriction or a throat in thecombustion boiler 2 that accelerates the combustion byproducts as theyflow from a primary combustion chamber 16 into the secondary combustionchamber 12. Finally, an exit section 18 is formed in one of thesidewalls 6 of the combustion boiler 2, above the inwardly taperingindentation 14 and adjacent the top wall 8. The exit section 18generally conveys the combustion byproducts of the combustion boiler 2to a further treatment apparatus or system, as is well known in the art,prior to discharging such combustion byproducts into the atmosphere.Since the further treating of the combustion byproducts prior todischarging the same into the atmosphere is well known in the art andper se forms no part of the present invention, a further discussionconcerning the same is not provided.

As only diagrammatically shown in FIG. 1 of the drawings, each one ofthe sidewalls 6 of the combustion boiler 2 includes an internal array ofa plurality of longitudinally arranged parallel conduits or pipes whichfacilitate the flow of a cooling fluid, e.g., cooling water, through thesidewalls 6 to remove heat thereform. The plurality of longitudinallyarranged parallel conduits or pipes generally extend from adjacent thetop wall 8 to adjacent the base wall 4. The cooling fluid is supplied toinlet(s) coupled to the plurality of longitudinally arranged parallelconduits or pipes and flows therethrough to absorb and remove heatgenerated within the primary and secondary combustion chambers 16, 12and absorbed by the sidewall 6 of the combustion boiler 2. The heatedfluid is then discharged, via a cooling fluid outlet(s) coupled thereto,and this heat fluid is typically used to drive a steam turbine 19 (onlydiagrammatically shown in FIG. 1), for example, and generate electricityin a conventional manner.

At least one sidewall 6, a pair of opposed sidewalls 6, or possibly oneor more corners of the boiler is provided with at least one, preferablya plurality of fuel supply duct(s) 20 which supply a desired fuel 22,e.g., finely ground coal, oil, gas, etc., mixed with an ample supply ofoxygen, from a fuel supply source 23 to the combustion boiler 2. Asshown in FIG. 1, two rows of fuel supply duct(s) 20 may be utilized forsupplying fuel 22 to the combustion boiler 2. The supplied fuel 22 isdischarged by an outlet of each one of the fuel supply duct(s) 20 intothe combustion boiler 2 toward a rear side of a lower region of theprimary combustion chamber 16 where the discharged fuel 22 rapidlyignites and is consumed. Generally, a high level of nitrogen oxides andcarbon monoxide are generated as the fuel is consumed and the nitrogenoxides and carbon monoxide have a tendency to collect adjacent the rearsurface of the combustion boiler 2. Alternatively, one or more rows offuel supply duct(s) 20 may be provided along an opposed sidewall 6 sothat the supplied fuel from the facing fuel supply ducts 20 intermixwith one another in a central area of the primary combustion chamber 16.This arrangement generally results in a high level of nitrogen oxidesand carbon monoxide in the central area of the combustion boiler 2.

The combustion boiler 2 typically operates at very high temperatures,e.g., between 2,800° and 3,300° F., and, as a result of suchtemperatures, the fuel 22 is substantially instantaneously consumed asit enters into the primary combustion chamber 16 of the combustionboiler 2. The combustion byproducts, resulting from combustion of thefuel 22, flow upward through the combustion boiler 2 due to theirelevated temperature.

As is well known in the art, the combustion byproducts resulting fromthe combustion of the fuel 22 generates nitrogen oxides which is harmfulto the environment and must be eliminated, as much as possible, prior toexhausting the combustion byproducts into the atmosphere. Carbonmonoxide is also generated as a byproduct. To facilitate a reduction orconversion of the nitrogen oxides into relatively harmless compositions(such as N₂ and H₂O, for example), a reducing agent 34 is typicallysupplied to an upper region 24, located adjacent to but below theinwardly tapering indentation 14, of the primary combustion chamber 16of the combustion boiler 2. The reducing agent 34 reduces the nitrogenoxides to N₂ and H₂O, and a further discussion concerning the same willfollow. Preferably, the reducing agent 34 is a mixture of water and oneor more concentrated reducing agents. The one or more concentratedreducing agent(s) is/are mixed with the water at a ratio of about 3% to15% of concentrated reducing agent to about 85% to 97% water. Examplesof suitable concentrated reducing agents 34 for use with the presentinvention and which are well known in the art are, for example, ammonia,ammonia salts, urea and urea prills.

The present invention, like other prior art combustion boilers, mayinclude a plurality of Selective Non-Catalytic Reduction (SNCR) ducts 26and 26′ (see FIGS. 1 and 3) which are typically equally spaced about aperimeter or periphery of the housing 10 to supply additional air, e.g.,oxygen, as well as a reducing agent 34 to the upper most region 24 ofthe primary combustion chamber 16 of the combustion boiler 2.Preferably, there are two sets of SNCR ducts, namely, a lower set ofprimary SNCR ducts 26 and an upper set of secondary SNCR duct 26′. Eachof these SNCR ducts 26, 26′ directs a SNCR stream 27 or 27′,respectively (FIG. 3 and 4), into the upper most region 24 of theprimary combustion chamber 16 of the combustion boiler 2. These SNCRstreams 27 and 27′ insure that there is ample supply of oxygen and thereducing agent 34 to facilitate substantially full and completeconsumption of all of the fuel 22 supplied by the fuel duct(s) 20 to theinterior of the combustion boiler 2.

The primary SNCR stream 27 encases and envelopes the central area of theupper most region 24 of the primary combustion chamber 16 and surroundsthe fuel 22 emitted by the fuel supply ducts 20. The primary SNCR stream27 generally flows at a rate of between 50 to 200 ft/sec, depending onthe depth of the boiler, and is typically designed to envelope the fuel22 emitted by the fuel supply ducts and assist with substantially fulland complete combustion of all of the fuel 22 prior to the combustionbyproducts being exhausted out through the exit section 18 of thecombustion boiler 2. The flow rate of the primary SNCR stream 27 willalso allow the reducing agent to flow towards the rear side wall andspread evenly within the rear third of the furnace. This distribution ofreducing agent within the primary SNCR stream 27 will allow for maximumdistribution of the reducing agent within the furnace with a minimumnumber of injectors. The reducing agent is generally completelyvaporized well before primary SNCR stream carries the reducing agenttowards the rear sidewall of the combustion boiler and more preferablythe reducing agent is completely vaporized about midway between thefront sidewall and the rear sidewall.

Preferably, the primary SNCR stream 27 is supplied at an elevatedtemperature, e.g., at a temperature of between about 300° to 800° F. andmore preferably between about 400° and 700° F., so that the primary SNCRstream 27 ensures that the reducing agent 34, added thereto, isvaporized and converted into a gaseous phase or state substantiallyinstantaneously as the reducing agent 34 is mixed with the primary SNCRstream 27 and enters a flue gas/reducing agent mixing zone 43 into uppermost region 24 of the primary combustion chamber 16 (see FIG. 5).

Alternatively, it is possible that the primary SNCR stream 27 may beeither room or external air which is supplied at ambient temperature,e.g., −50° F. to 120° F. for example depending upon the temperature ofthe ambient air. According to this embodiment, the reducing agent 34 isgenerally only vaporized and converted into a gaseous phase or statesubstantially instantaneously upon the reducing agent 34 entering intoand mixing with the gases of combustion chamber 16.

A reducing agent injection device 33 (FIG. 6) has a spray nozzle 35which is located within the primary SNCR duct 26 and a supply end of thereducing agent injection device 33 is connected to a reducing agentsupply source 54 by a reducing agent conduit 56 and a supply pump 58.The supply pump 58 draws the reducing agent 34 from the reducing agentsupply source 54, via the reducing agent conduit 56, and conveys thereducing agent 34 to the spray nozzle 35 of the reducing agent injectiondevice 33. The spray nozzle 35 then sprays a very fine mist 41 of thereducing agent 34 directly into the primary SNCR stream 27 passingtherethrough. The sprayed reducing agent is of a very fine mist 41generally having a particle size of between 1 microns and 40 microns ormore preferably having a particle size of between 15 microns and 35microns. As shown in FIG. 2, the injection device 33 preferably has aspray discharge angle α ranging from about 60° to about 140°. This verysmall particle size in combination with the temperature of the primarySNCR stream 27 and the high energy of the overfire air stream allows forrapid evaporation of the reducing agent/water mixture to occur withinabout 0.01 to 0.05 seconds after the reducing agent/water mixturecontacts the relatively high temperature air of the primary SNCR stream27. The mixture of the vaporized or gaseous reducing agent 34 and theheated primary SNCR stream 27 is then discharged into the upper mostregion 24 of the primary combustion chamber 16 in a flue gas/reducingagent mixing zone 43. In a preferred form of the invention, the reducingagent 34 is added to the primary SNCR stream 27 so that the reducingagent 34 comprises, for example, generally about 1% to 25% of theprimary SNCR stream 27 or more preferably about 2.5% to 7.5% of theprimary SNCR stream 27.

The present invention, in addition, may include a plurality of secondarySNCR ducts 26′ (see FIG. 4) which are equally spaced about a perimeteror periphery of the secondary combustion chamber 12 to supply additionair, e.g., oxygen, and a reducing agent 34 to the secondary combustionchamber 12 and further insure that there is ample supply of oxygen andthe reducing agent in the secondary combustion chamber 12 and furtherfacilitate substantially full and complete consumption of all of thefuel 22 and reduction of the byproducts prior to the byproducts beingexhausted from the combustion boiler 2 by the exit section 18. Thesecondary SNCR stream 27′ supplies additional air and the reducing agent34 to insure maximum efficiency of the combustion boiler 2 as well asreduce the amount of unconsumed fuel 22 which may possibly be exhaustedout through the exit section 18 of the combustion boiler 2. Thesecondary SNCR stream 27′ encases and envelopes the byproducts as theyflow into the secondary combustion chamber 12. The secondary SNCR stream27′ flows at a rate of typically between 50 to 200 ft/sec, depending onthe depth of the boiler, and is typically designed to envelope thecombustion byproducts as well as any residual fuel 22 and further assistwith substantially full and complete combustion of all of the fuel 22and reduction of the byproducts prior to the combustion byproducts beingexhausted out through the exit section 18 of the combustion boiler 2.The flow rate of the secondary SNCR stream 27′ will also allow thereducing agent 34 to flow towards the rear wall and spread evenly withthe rear third of the furnace. This distribution of reducing agentwithin the secondary SNCR stream 27′ assists with maximum distributionof the reducing agent 34 within the furnace with a minimum number ofinjectors.

Preferably, the secondary SNCR stream 27′ is supplied at an elevatedtemperature, e.g., at a temperature of between 300° and 800° F., or morepreferably at a temperature of between 400° and 700° F., and at arelatively high velocity, e.g., at a velocity typcially ranging fromabout 50 to about 200 ft/sec. The high energy of the secondary SNCRstream 27′ ensures that the reducing agent 34, added to the secondarySNCR stream 27′, is evaporated and converted into a gaseous phase orstate substantially instantaneously as the reducing agent 34 is mixedwith the secondary SNCR stream 27′ and enters into of the secondarycombustion chamber 12 in a flue gas/reducing agent mixing zone 43′. Areducing agent injection device 33 has a spray nozzle 35 which islocated within the secondary SNCR duct 26′ and a supply end of thereducing agent injection device 33 is connected to the reducing agentsupply source 54 by a second reducing agent conduit 56 and a second pump58. The second pump 58 pumps the reducing agent 34 from the reducingagent supply source 54 to the spray nozzle 35 of the reducing agentinjection device 33, via the second reducing agent conduit 56. The spraynozzle 35 then sprays a very fine mist 41′ of the reducing agent 34directly into the secondary SNCR stream 27′ passing therethrough. Thesprayed reducing agent 34 in the very fine mist 41′ generally has aparticle size of between 1 microns and 40 microns or more preferably hasa particle size of between 15 microns to 35 microns. The injectiondevice 33 preferably has a spray discharge angle α ranging from about60° to about 140° (FIG. 2). The fine mist of the liquid reducing agent34 is substantially instantaneously gasified or vaporized as soon as itcontacts the relatively high temperature secondary SNCR stream 27′. Thatis, the particle size in combination with the temperature of thesecondary SNCR stream 27′ and high energy of the overfire air streamallows for evaporation of the reducing agent/water mixture to occurwithin about 0.01 to 0.05 seconds after the reducing agent contacts therelatively high temperature secondary SNCR stream 27′. The mixture ofthe vaporized or gaseous reducing agent 34 and the heated secondary SNCRstream 27′ is then discharged into the secondary combustion chamber 12in the flue gas/reducing agent mixing zone 43′. In a preferred form ofthe invention, the reducing agent 34 is added to the secondary SNCRstream 27′ so that the reducing agent 34 comprises, for example,generally about 1% to about 25% of the secondary SNCR stream 27′.

One difference between the present invention and the prior art systemsheretofore available is that an initial overfire air stream 31 (see FIG.5) is provided and this initial overfire air stream 31 is suppliedbetween the fuel supply ducts 20 and the primary SNCR ducts 26. Theinitial overfire air stream 31 can be divided into several distinctoverfire air streams, namely, an initial primary overfire air stream 30and an initial secondary overfire air stream 32. The initial primaryoverfire air stream 30 is similar to conventional prior art overfire airstreams as it is merely a source of additional air or oxygen designed tosurround, encase and envelope a central area of a lower region of theprimary combustion chamber 16 of the combustion boiler 2 which containsthe consumed fuel emitted by the one or more fuel supply ducts 20. Theinitial secondary overfire air stream 32, on the other hand, contains agaseous or vaporized reducing agent 34 therein and the initial secondaryoverfire air stream 32 is emitted and designed to surround, encase andenvelope both the initial primary overfire air stream 30 and theconsumer fuel 22 emitted by the one or more fuel supply ducts 20.

To facilitate dividing or partitioning the overfire air stream into theinitial primary and initial secondary overfire air stream streams 30,32, the discharged outlet 36 of the at least one, preferably all, of theinitial overfire air supply ducts 39 has a discharge nozzle 40 (see FIG.5) which has an internal wall, surface or some other internal dividingor partitioning member 42 that physically and aerodynamically dividesthe interior space of the discharge nozzle 40 into a larger primary flowpassage 44 and a smaller secondary flow passage 46. Preferably, theentire initial overfire air stream is divided such that greater than 50percent of the initial overfire air stream 31, supplied by the initialoverfire air duct 39 to the discharge nozzle 40, is channeled ordiverted and forms the initial primary overfire air stream 30 while theremainder of the initial overfire air stream 31 supplied by the initialoverfire air duct 39 to the discharge nozzle 40, i.e., less than 50percent of the initial overfire air stream 31, is channeled or divertedand forms the initial secondary overfire air stream 32.

It is to be appreciated that the initial overfire air stream 31, priorto reaching the discharge nozzle 40, is typically a single stream ofair. Preferably, the overfire air, supplied by the initial overfire airsupply duct 39, flows at a rate of between 50 to 200 ft/sec, dependingon the depth of the boiler, and is at an elevated temperature, e.g., ata temperature of between about 300° and 800° F., and more preferably ata temperature of between about 400° and 700° F. or so, and the highenergy of the overfire air stream ensures that the reducing agent 34,added to the initial secondary overfire air stream 32, is converted intoa gaseous phase or state substantially instantaneously as the reducingagent 34 is mixed with the initial secondary overfire air stream 32 andenters into mid region 25 of the primary combustion chamber 16 of thecombustion boiler 2. The flow rate of the initial overfire air stream 31will also allow the reducing agent added to the initial secondaryoverfire air stream 32 to flow towards the back wall and spread evenlyto the back third of the furnace. This distribution of reducing agentwithin the initial secondary overfire air stream 32 will allow formaximum distribution within the furnace with a minimum number ofinjectors.

A reducing agent injection device 52 has a spray nozzle 50 (see FIG. 5)which is located within the secondary flow passage 46 of the dischargenozzle 40 and a supply end of the reducing agent injection device 52 isconnected to a reducing agent supply source 54 by a reducing agentconduit 56 and a pump 58. The pump 58 pumps the reducing agent 34 fromthe reducing agent supply source 54, via the reducing agent conduit 56,to the spray nozzle 50 of the reducing agent injection device 52. Thespray nozzle 50 then sprays a very fine mist of the reducing agent 34directly into the initial secondary overfire air stream 32 passingthrough the secondary flow passage 46 of the discharge nozzle 40. Thesprayed reducing agent in the very fine mist generally has a particlesize of between 1 microns and 40 microns or more preferably has aparticle size of between 15 microns and 35 microns. The injection device52 preferably has a spray discharge angle a ranging from about 60° toabout 140°. The fine mist of the liquid reducing agent 34 issubstantially instantaneously gasified or vaporized as soon as itcontacts the relatively high temperature initial secondary overfire airstream 32. The mixture of the vaporized or gaseous reducing agent 34 andthe heated initial secondary overfire air stream 32 is then dischargedfrom the secondary flow passage 46 of the discharge nozzle 40 into midregion 25 of the primary combustion chamber 16 where the initialsecondary overfire air stream 32 surrounds, encases and envelops theprimary overfire air stream 30. Preferably, the outlet of the secondaryflow passage 46 of the discharge nozzle 40 has an upwardly inclined ordirected deflector 60 which is designed to deflect or direct the initialsecondary overfire air stream 32, as it enters the combustion boiler 2,toward an upper region of the primary combustion chamber 16.

The particle size of the reducing agent in combination with thetemperature and the high energy of the initial secondary overfire airstream 32 allows for rapid evaporation of the reducing agent/watermixture to occur within about 0.01 to 0.05 seconds after the reducingagent contacts the relatively high temperature initial secondaryoverfire air stream 32. As soon as the mixture of the vaporized orgaseous reducing agent 34 and the heated initial secondary overfire airstream 32 enters into the combustion boiler 2, the vaporized or gaseousreducing agent 34 is then permitted to disperse and intermix with thenitrogen oxides and reduce the nitrogen oxides to relatively harmlesscomponents, such as N₂ and H₂O, which can then be discharged orexhausted into the atmosphere. In one form of the invention, thereducing agent 34 is added to the initial secondary overfire air stream32 so that the reducing agent 34 comprises, for example, generally about0.1% to about 1% of the initial secondary overfire air stream 32.

During operation of the combustion boiler 2, the fuel 22 is supplied bythe fuel supply ducts 20 to the interior combustion chamber of thecombustion boiler 2 where the fuel 22 readily ignites and combusts intoits conventional combustion byproducts. The combustion byproducts,resulting from combustion of the fuel 22, rise within the primarycombustion chamber 16 and intermixes with the initial primary overfireair stream 30 which generally surrounds and encases the burning fuel.Any remaining uncombusted or unconsumed fuel 22 then intermixes with theinitial primary overfire air stream 30 and the oxygen contained thereinfacilitates further combustion of any remaining fuel as this fuel flowstoward the exit section 18 of the combustion boiler 2. The additionalair of the initial primary overfire air stream 30 substantially ensurescomplete combustion of all of the supplied fuel 22 prior to thecombustion byproducts exiting via the exit section 18 of the combustionboiler 2.

The combustion byproducts of the fuel continue to flow toward the throatof the combustion boiler 2 and as such combustion byproducts flow towardthe exit section 18, they intermix with the initial secondary overfireair stream 32 which is designed to completely surround, encase andenvelope both the initial primary overfire air stream 30 as well as thefuel 22 located generally in the mid region 25 of the combustion boiler2. As the combustion byproducts, resulting from the combustion processof the fuel 22, flow toward the exit section 18 of the combustion boiler2, the combustion byproducts intermix with the initial secondaryoverfire air stream 32. As this intermixing occurs, the gaseous orvaporized reducing agent 34 contained in the initial secondary overfireair stream is permitted to react with the nitrogen oxides and reduce thenitrogen oxides into N₂ and H₂O.

It is to be appreciated that within the combustion boiler 2, the regionsor boundary between the combustion zone and the initial primary overfireair stream 30 or between the primary overfire air stream 30 and thesecondary overfire air region 32 are not well defined and tend to move,vary or overlap somewhat during operation of the combustion boiler 2.Those regions or zones are described with this specification merely tohelp facilitate understanding of the present invention.

As the combustion byproducts, the initial primary overfire air stream,the initial secondary overfire air stream and the primary combustionchamber overfire air stream all flow toward the exit section 18 of thecombustion boiler 2, these streams and byproducts are all accelerateddue to the constriction formed by the throat of the combustion boiler 2.This acceleration of the combustion byproducts and the initial overfireair streams 30, 32 and the primary combustion chamber overfire airstream 27 facilitates a more thorough and complete mixing of theoverfire air streams and the combustion byproducts and assists withfurther reducing the nitrogen oxides. The constriction formed by thethroat of the combustion boiler 2 accelerates the overfire air streams30, 32, the primary combustion chamber overfire air stream 27 and thecombustion byproducts toward the top wall 8 of the combustion boiler 2where the flow path of the streams and the combustion byproducts impingeagainst and turn sharply prior to exiting the combustion boiler 2through the exit section 18. The secondary combustion chamber 12facilitates intimate mixing of all of the sources of the overfire airstreams, any remaining fuel, the combustion byproducts and any remainingreducing agent 34 to assist with minimizing the amount of nitrogenoxides which is exhausted out through the discharge port of thecombustion boiler 2.

Field testing of the above method and apparatus has found increasedreduction in NO_(x). A known 50 MW coal fired front wall fired boiler,injecting large droplets of reducing agent/water mixture through threefront wall overfire ports resulted in only a 9% reduction in the NO_(x)emissions. However, by utilizing the method and apparatus of the presentinvention, which produces reducing agent droplet sizes of about 25microns, the smaller droplets were more rapidly and quickly evaporatedand/or vaporized and this, in turn, led to a 30% reduction in the NO_(x)emissions. The inventors have discovered that by evaporating, vaporizingand/or gasifying the reducing agent droplets quickly, upon mixing withthe SNCR stream, such rapid evaporation results in better distributionof reducing agent within the furnace and this leads to a more completereduction in the generate nitrogen oxides.

In another example, with a 180 MW coal fired front wall fired boiler,the rapid vaporization and expansion of the reducing agent facilitatesan improved dispersion of the reducing agent within the overfire airsteam which, in turn, greatly increases the dispersion and coverage ofthe reducing agent within the combustion boiler. This rapid vaporizationand expansion of the reducing agent facilitates an improved and morecomplete intermixing of the reducing agent within the overfire air steamwhich, in turn, leads to an improved and more complete intermixing ofthe reducing agent, contained within the overfire air steam, with thecombustion gases located in the combustion boiler. That is, since thevolume of the reducing agent increase sooner and more rapidly than anyother conventional method or apparatus, e.g., within the overfire airduct and/or immediately upon entering the combustion boiler, and thisleads to a more complete intermixing of the reducing agent with thecombustion gases and a reduction in the NO_(x) emissions. The aboveexemplifies the importance of vaporizing or evaporating the reducingagent droplets in the SNCR stream to increase the coverage area withinthe combustion boiler and increase the reduction in the NO_(x)emissions.

An important aspect of the present invention relates to spraying ordispensing the reducing agent in a fine mist or small enough particlesize, e.g., from about 1 to about 40 microns in size, so that theliquified reducing agent is substantially instantaneously vaporized,either prior to or immediately after entering into the combustion boilerand the additional overfire air facilitates conveying or transferringthis vaporized reducing agent and mixing with the nitrous oxide toconvert the same to water and carbon dioxide, for example. The inventorsbelieve that it is a combination of generating sufficiently smallreducing agent droplets which can substantially instantaneously vaporizeand thus are readily able to intimately mix and react with the nitrousoxide that results in the increase in the amount of nitrous oxideremoved from the exhaust stream prior to exhausting the same from theboiler.

The inventors believe that three factors are important in order tofacilitate intimate mixing of the reducing agent with the nitrogenoxides contained within the combustion boiler. Firstly, the reducingagent must be substantially in vaporized form, either prior to orimmediately after entering the combustion boiler, i.e., the reducingagent must be in a gaseous or vaporized state either before entering thecombustion boiler or within about 0.1 second after the reducing agententers the combustion boiler. Secondly, the reducing agent must be of asufficient concentration so that the reducing agent is readily availableto react with the nitrogen oxides contained within the combustion boilerwhile not being of an excess concentration so that some of the reducingagent is unable to react with the nitrogen oxides, contained within thecombustion boiler, prior to being exiting from the combustion boiler.That is, the reducing agent should constitute a concentration of betweenabout 3 percent and about 5 percent of the overfire air stream. Thirdly,the reducing agent must be sprayed from a nozzle having a dispersionangle of preferably between 60 degrees and 140 degrees. The inventorshave determined that if the spray angle is excessive, e.g., over 140degrees, for example, the concentration of the reducing agent within theoverfire air stream is sufficiently diluted and thus leads to lessreduction of the nitrogen oxides compounds. Alternatively, if the nozzledispensing angle is less than about 60 degrees, the concentration of thereducing agent within the overfire air stream is too concentrated and aportion of the reducing agent may have a tendency to be exhausted fromthe combustion boiler without reacting with any of the nitrogen oxidescontained within the combustion boiler.

Preferably, the reducing agent is directed at and supplied to a centralregion or area of the combustion boiler above the location where thefuel is supplied to the combustion boiler but below the throat of thecombustion boiler. Preferably, the reducing agent is substantiallycompletely evaporated/gasified by the time that the reducing agent isconveyed, by the overfire air stream, and reaches the rear wall of thecombustion boiler and more preferably, the reducing agent issubstantially completely evaporated/gasified by the time that thereducing agent is conveyed, by the overfire air stream, and reaches thecentral region of the combustion boiler. The addition of the reducingagent in the above noted concentration and area has a tendency tofacilitate more intimate contact and complete mixing of the reducingagent with the nitrogen oxides contained within the combustion boilerand thereby achieves a maximum reduction in the nitrogen oxidescontained in the exhaust stream.

As used in the above description and the appended claims, the term“vaporize” is used interchangeable with the terms “evaporate” and/or“gasify”. These terms all mean that the reducing agent is rapidlyconverted into a gaseous phase or state substantially instantaneouslyonce the reducing agent is mixed with the overfire air stream or, at thevery latest, substantially immediately after the reducing agent enteringinto the combustion boiler, i.e., prior to the reducing agent contactingor reaching the rear wall of the combustion boiler.

The discharge outlet of the spray nozzle, for spraying the very finemist of reducing agent, is typically located closely adjacent theinterface between the overfire air duct and the wall of the combustionboiler, e.g., within a few inches to a few feet of the interface.

Since certain changes may be made in the above described method ofimproving reduction in the amount of nitrogen oxides discharged into theatmosphere from a combustion boiler and a combustion boiler forachieving the same, without departing from the spirit and scope of theinvention herein involved, it is intended that all of the subject matterof the above description or shown in the accompanying drawings shall beinterpreted merely as examples illustrating the inventive concept hereinand shall not be construed as limiting the invention.

1. A method of minimizing discharge of nitrogen oxides from a combustionboiler, the method comprising the steps of: spraying a reducing agentinto an overfire air stream; supplying the overfire air stream with thesprayed reducing agent to the combustion boiler; vaporizing the sprayedreducing agent at least within about 0.1 seconds of the sprayed reducingagent entering the combustion boiler; and reacting the vaporizedreducing agent with the nitrogen oxides within the combustion chamber toreduce the nitrogen oxides and minimize discharge of nitrogen oxidesfrom the combustion boiler.
 2. The method of claim according to claim 1,further comprising the step of using one of ammonia, ammonia salts, ureaand urea prills as the reducing agent.
 3. The method of claim accordingto claim 1, further comprising the step of spraying the reducing agentinto the overfire air stream at a discharge angle of between about 60°to about 140°.
 4. The method of claim according to claim 1, furthercomprising the step of spraying the reducing agent into the overfire airstream such that a concentration of the reducing agent within theoverfire air stream is between about 3 to about 5 percent.
 5. The methodof claim according to claim 1, further comprising the step of dividingat least one initial overfire air duct, for supplying an initialoverfire air stream to the combustion boiler and facilitatesubstantially complete combustion of fuel supplied to the combustionboiler, into an initial primary overfire air stream and an initialsecondary initial overfire air stream, and adding the reducing agentonly to the initial secondary initial overfire air stream, but not theinitial primary initial overfire air stream, prior to the secondaryinitial overfire air stream discharging into the combustion chamber. 6.The method of claim according to claim 5, further comprising the step ofusing at least 50 percent of the initial overfire air stream, suppliedby the at least one initial overfire air duct, to form the initialprimary overfire air stream and using a remainder of the initialoverfire air stream, supplied by the at least one initial overfire airduct, to form the secondary initial overfire air stream.
 7. The methodof claim according to claim 5, further comprising the step of providinga reducing agent injection nozzle in the initial secondary overfire airstream for injecting the reducing agent, in liquid form, into theinitial secondary air stream and vaporizing the reducing agent prior tothe initial secondary overfire air stream entering the combustionchamber.
 8. The method of claim according to claim 5, further comprisingthe step of heating the initial overfire air stream to a temperature ofbetween 300 and 800 degrees F. prior to the initial overfire air streambeing divided into the primary and the secondary initial overfire airstreams.
 9. The method of claim according to claim 1, further comprisingthe step of spacing a plurality of primary combustion chamber overfireair ducts about a periphery of the combustion boiler for supplyingaddition air to a mid region of the primary combustion chamber andfacilitating a substantially complete consumption of the fuel suppliedby the at least one fuel supply duct to the combustion boiler.
 10. Themethod of claim according to claim 1, further comprising the steps ofsupplying a cooling fluid to a wall of the combustion boiler to absorband remove heat generated within the combustion chamber, and utilizingthe heated fluid to drive a steam turbine and generate electricity. 11.The method of claim according to claim 1, further comprising the step offorming the reducing agent by mixing one of ammonia, ammonia salts, ureaand urea prills with water in a ratio of about 3% to 15% of one ofammonia, ammonia salts, urea and urea prills with about 85% to 97% ofwater.
 12. The method of claim according to claim 1, further comprisingthe step of spraying the reducing agent to have a particle size ofbetween 1 micron and 40 microns.
 13. The method of claim according toclaim 1, further comprising the step of adding the reducing agent to theoverfire air stream such that the reducing agent comprises generallyabout 1% to 25% of the overfire air stream.
 14. The method of claimaccording to claim 1, further comprising the step of adding the reducingagent to the overfire air stream such that the reducing agent comprisesgenerally about 2.5% to 7.5% of the overfire air stream.
 15. Acombustion boiler for combusting fuel and generating heat, thecombustion boiler comprising: a housing defining an internal combustionchamber therein; at least one fuel supply duct connected to thecombustion boiler for supplying fuel to the combustion chamber; and atleast one initial overfire air duct for supplying an initial overfireair stream to the combustion chamber to facilitate complete combustionof the fuel supplied to the combustion boiler; wherein the at least oneinitial overfire air duct is divided into an initial primary overfireair stream and an initial secondary initial overfire air stream, and areducing agent is added only to the initial secondary initial overfireair stream, but not the initial primary initial overfire air stream,prior to the secondary initial overfire air stream discharging into thecombustion chamber.
 16. The combustion boiler according to claim 15,wherein the housing comprises a base wall, a sidewall and a top wallwith an exit section formed in the sidewall adjacent the top wall, andan indentation is formed in the sidewall of the housing to form a throatwhich accelerates combustion byproducts and the overfire air and anyresidual reducing agent as the combustion byproducts, the overfire airand any residual reducing agent flow from a primary combustion chambertoward a secondary combustion chamber located above the indentation inthe combustion boiler.
 17. The combustion boiler according to claim 15,wherein the reducing agent added to the initial secondary initialoverfire air stream is selected from the group comprising ammonia,ammonia salts, urea and urea prills.
 18. The combustion boiler accordingto claim 16, wherein the reducing agent injection nozzle has a dischargespray angle of between about 60° to about 140° and a plurality of fuelsupply ducts supply fuel to the combustion chamber where the fuelignites and is consumed upon operation of the combustion boiler.
 19. Thecombustion boiler according to claim 15 wherein the initial overfire airstream is heated to a temperature of between 300 and 800 degrees F.prior to the initial overfire air stream being divided into the primaryand the secondary initial overfire air streams.
 20. The combustionboiler according to claim 15, wherein the housing comprises a base wall,a sidewall and a top wall with an exit section formed in the sidewalladjacent the top wall, and an indentation is formed in the sidewall ofthe housing to form a throat which accelerates combustion byproducts andthe overfire air and any residual reducing agent as the combustionbyproducts, the overfire air and any residual reducing agent flow from aprimary combustion chamber toward a secondary combustion chamber locatedabove the indentation in the combustion boiler; the reducing agent addedto the initial secondary initial overfire air stream is selected fromthe group comprising ammonia, ammonia salts, urea and urea prills; thereducing agent injection nozzle has a discharge spray angle of betweenabout 60° to about 140° and a plurality of fuel supply ducts supply fuelto the combustion chamber where the fuel ignites and is consumed uponoperation of the combustion boiler; and the initial overfire air streamis heated to a temperature of between 300 and 800 degrees F. prior tothe initial overfire air stream being divided into the primary and thesecondary initial overfire air streams.